1. Technical Field
The present invention relates to a low-pass filter, a constant voltage circuit, and a semiconductor integrated circuit including the same, and more particularly, to a low-pass filter and a constant voltage circuit for use in ultra-low noise constant voltage regulation which can be integrally formed on a single semiconductor substrate, and a semiconductor integrated circuit including such a voltage regulator with a low-pass filter incorporated therein.
2. Discussion of the Background
Electronic low-pass filters are used in various semiconductor circuits which eliminate high frequencies above a given cutoff frequency to provide accurate signals free from high-frequency noise. One typical application is in voltage regulation, where a low-pass filter is connected between a reference voltage generator output terminal and a regulator output terminal to filter out flicker or 1/f noise inherent in the semiconductor device from a reference voltage based on which an output voltage is regulated.
FIG. 1 is a circuit diagram schematically illustrating a constant voltage circuit 100 employing a conventional, resistance-capacitance low-pass filter 110 consisting of a resistor R111 and a capacitor C111 connected in series.
As shown in FIG. 1, the constant voltage circuit 100 is a series regulator that regulates an input voltage Vin input to an input terminal IN to output a constant output voltage Vout to an output terminal OUT, including a bipolar, output transistor M111 connected between the input and output terminals IN and OUT, a resistor R112 and a Zener diode ZD connected in series between the input terminal IN and ground to form a reference node Nref therebetween, and an error amplifier 111 with a non-inverting input connected to the node Nref through the RC low-pass filter 110, an inverting input connected to the output terminal OUT, and an output connected to a base terminal of the output transistor M111.
During operation, the Zener diode ZD generates a reference voltage Vref at the reference node Nref for input to the non-inverting input of the error amplifier 111, which compares the reference voltage Vref against the output voltage Vout input to its inverting input to output a regulator control signal that controls the base current of the output transistor M111 so as to maintain the output voltage Vout equal to the reference voltage Vref.
Interposed between the reference node Nref and the non-inverting input of the error amplifier 111, the low-pass filter 110 has the series circuit of the resistor R111 and the capacitor C111 connected across the node Nref and ground. The resistor R111 and the capacitor C111 are provided with particular resistance and capacitance scaled to yield an appropriate cutoff frequency rated in the range of below one to several hertz (Hz) depending on specific requirements of the voltage regulator. For example, a cutoff frequency of approximately 1 Hz, which is required for proper filtering of 1/f noise, can be obtained in the low-pass filter 110 with the resistor R111 having a value of 1 megaohms (MΩ) and the capacitor C111 having a value of 1 microfarad (μF).
The conventional low-pass filter 110 is not practical where the cutoff frequency desired is very low. This is because, in practice, all the components of the filtering circuit are constructed on a single semiconductor substrate for integration into a monolithic IC, which imposes limits on the physical sizes and therefore the values of both the resistor and the capacitor in use.
For example, consider a case where the capacitor C111 has its value limited to below 100 picofarads (pF). With such a small capacitance, obtaining a cutoff frequency of 1 Hz requires a resistance of 10 gigaohms (GΩ) or higher of the resistor R111, which is technically difficult to form on a single semiconductor substrate on which the capacitor C111 is disposed. Thus, the conventional low-pass filter 110 is implemented with at least one of the resistor R111 and the capacitor C111 built as a discrete component external from the integrated circuit, making the implementation less successful than desired.
The problem of the conventional low-pass filter 110 may be overcome by replacing the resistor R111 with a transistor operated with no gate bias voltage applied thereto. Compared to a simple resistor, a zero-biased transistor provides an extremely high impedance relative to its size, allowing for obtaining a low cutoff frequency with a reasonably small capacitance without requiring large space in the semiconductor circuit.
FIG. 2 is a circuit diagram schematically illustrating a constant voltage circuit 200 employing a low-pass filter 210 consisting of a zero-biased transistor M211 and a capacitor C211 connected in series.
As shown in FIG. 2, the constant voltage circuit 200 is a series regulator that regulates an input voltage Vin input to an input terminal IN to output a constant output voltage Vout to an output terminal OUT, including a p-channel metal-oxide semiconductor (PMOS) transistor M201 connected between the input and output terminals IN and OUT, a reference voltage generator 221, and a reference voltage amplification circuit formed of an operational amplifier 212 with an inverting input connected to a node between a pair of resistors R213 and R214 connected in series, a non-inverting input connected to the reference voltage generator 221, and an output connected to its non-inverting input through the resistor R213 to form an amplified reference node Nref, as well as a buffer amplifier 211 with a non-inverting input connected to the node Nref through the low-pass filter 210, a non-inverting input connected to the output terminal OUT, and an output connected to a gate terminal of the output transistor M201.
During operation, the reference voltage generator 221 generates a reference voltage Vref for input to the reference amplification circuit, which then generates an amplified reference voltage at the reference node Nref for input to the inverting input of the buffer amplifier 211. The buffer amplifier 211 compares the amplified reference voltage against the output voltage Vout input to its non-inverting input to generate a regulator control signal that controls the operation of the output transistor M201 so as to maintain the output voltage Vout equal to the amplified reference voltage.
Interposed between the amplified reference node Nref and the input of the buffer amplifier 211, the low-pass filter 210 has the zero-biased transistor R211 and the capacitor C211 connected in series across the node Nref and ground. The transistor M211 is a PMOS transistor with its gate and source terminals connected together to exhibit an extremely high impedance, higher than that obtained with a simple resistor. Using the zero-biased transistor M211 as an impedance allows for implementing the low-pass filter 210 on a single integrated circuit, with a sufficiently low cutoff frequency even where the capacitor C211 is of a small value.
Although effective in providing a low cutoff frequency with a relatively small circuit, the low-pass filter 210 depicted above has a drawback. That is, variations in the cutoff frequency can occur due to variations in the impedance of the zero-biased transistor M211, which has variations in physical properties from one transistor to the next caused by manufacturing process inconsistencies or environmental changes that are difficult to control and eliminate completely, resulting in reduced accuracy and stability of the low-pass filter 210. To address this problem, several methods have been proposed to stabilize the impedance of the biased transistor in the low-pass filter 210.
FIG. 3 is a circuit diagram of another conventional low-pass filter 210a for use in the constant voltage circuit 200, shown with an input terminal LPIN for connection with the reference node Nref and an output terminal LPOUT for connection with the error amplifier input.
As shown in FIG. 3, the low-pass filter 210a has the series circuit of the PMOS transistor M211 and the capacitor C211 arranged with an additional, PMOS transistor M212 and a current source I211 connected in series between the input terminal LPIN and ground. The two PMOS transistors M211 and M212 have their source terminals connected together and their gate terminals connected together and to the drain of the transistor M212 which is connected to the current source I211. With the transistors M211 and M212 thus forming a current mirror, the transistor M211 conducts an amount of current proportional to a current i211 supplied to the transistor M212 from the current source I211.
In such a configuration, varying the amount of current i211 allows adjustment of the impedance of the biased transistor M211 to a desired value lower than that obtained with no bias voltage applied to the transistor. The ability to adjust the transistor impedance enables the low-pass filter 210a to operate with a desired cutoff frequency regardless of manufacturing process inconsistencies and environmental changes.
FIG. 4 is a circuit diagram illustrating still another conventional low-pass filter 210b for use in the constant voltage circuit 200.
As shown in FIG. 4, the low-pass filter 210b includes, in addition to the capacitor C211, the PMOS transistors M211 and M212, and the current source I211, another current mirror formed of a pair of n-channel metal-oxide semiconductor (NMOS) transistors M213 and M214 inserted between the current source I211 and the current mirror of the transistors M211 and M212. The NMOS transistor M214 is sized twenty-five times larger than the NMOS transistor M213, and the PMOS transistor M212 approximately nine hundred sixty times larger than the PMOS transistor M211, so that the amount of current supplied to the transistor M211 through the two current mirrors is approximately 1/24,000 times smaller than the current i211 supplied from the current source I211.
In addition to being capable of adjusting the impedance of the biased transistor M211, provision of the dual-current mirror circuit allows the low-pass filter 210b to precisely adjust the current through the transistor M211 relative to the supplied current i211, compared to the circuit depicted in FIG. 3 which requires precise control of an extremely small and consistent current i211 supplied from the current source I211 to obtain a sufficiently high impedance of the transistor M211.
Although obtaining higher accuracy and stability of the transistor impedance compared to those depicted in FIGS. 2 and 3, even the improved circuit 210b is still susceptible to variations where the current source I211 itself has variations resulting from manufacturing process inconsistencies or environmental changes. Variations in the supplied current i211 affect the gate bias voltage of the transistor M212 that is the gate bias voltage of the transistor M211, resulting in significant variations in the impedance of the transistor M211 and concomitant variations in the cutoff frequency of the low-pass filter 210b. 